Gay Men in Twin Study

Published: December 17, 1991

CHICAGO, Dec. 16—
A new study of twins provides the strongest evidence yet that homosexuality has a genetic basis, researchers say, though they say other factors like social conditioning may be important.

The study, published in the December issue of The Archives of General Psychiatry, adds to evidence that sexual orientation does not result from a maladjustment or moral defect, one author said.

"We found 52 percent of identical twin brothers of gay men also were gay, compared with 22 percent of fraternal twins, compared with 11 percent of genetically unrelated brothers," said J. Michael Bailey, an assistant professor of psychology at Northwestern University in Evanston, "which is exactly the kind of pattern you would want to see if something genetic were going on." By "unrelated," Dr. Bailey was referring to brothers by adoption.

"The genetically most similar brothers were also the ones most likely to be gay, by a large margin," he added.

Identical twins are genetic clones, having developed in the womb from a single egg that split after being fertilized by a single sperm. Fraternal twins develop simultaneously from two separate eggs fertilized by two separate sperm cells, making them only as similar as non-twin siblings.

"This is the first real genetic study of sexual orientation in about 40 years," said Dr. Bailey, whose co-author was Dr. Richard C. Pillard, a psychiatry professor at Boston University School of Medicine.

Dr. Bailey estimated that the degree of the genetic contribution to homosexuality could range from 30 percent to more than 70 percent, depending on varying assumptions about the prevalence of homosexuality and how well the sample represents twins in the general population.

Gregory Carey, an assistant professor of psychology at the University of Colorado, called the work very important. "I'm not terribly surprised at the conclusions," he said. "I think they're very well founded. Some of the earlier evidence suggested there was genetic effect, but the studies were not well done. This is something that really sort of clinches it."